The present invention relates to clock extraction from a PCM (pulse code modulation) signal particularly but not exclusively for use in repeaters for submerged transmission systems.
Where the clock extraction circuit has a passive tank or resonator circuit which is driven by a clock signal extracted from the incoming digital data stream, and where the data signal is a non-return-to-zero signal (NRZ), with the kind of bit patterns likely to be received in, for example, a submerged digital data system, the clock signal may tend to die out.